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a=xqWC_Br6kY4A:10 a=IkcTkHD0fZMA:10 a=vUbySO9Y5rIA:10 a=GoEa3M9JfhUA:10 a=VkNPw1HP01LnGYTKEx00:22 a=cXQAnHGZ2eiM_jKl3LIA:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 On 12/01/2026 16:57, David wrote: > Hi > > I'm trying to use `bpf_strstr` in a program that's running on kernel 6.18.2, > but I'm getting the following error on load: > >> failed to find BTF for extern 'bpf_strstr' [53] section: -2 > > A minimal reproducer for this is: > > ``` > extern int bpf_strstr(const char *s1__ign, const char *s2__ign); I think you need to add "__ksym __weak";" here i.e. extern int bpf_strstr(const char *s1__ign, const char *s2__ign) __ksym __weak; If these aren't already defined you'll need: #ifndef __ksym #define __ksym __attribute__((section(".ksyms"))) #endif #ifndef __weak #define __weak __attribute__((weak)) #endif Most of the examples use a bpftool-generated vmlinux.h which has kfunc declarations of that form; if you want to use a generated vmlinux.h (created from the running kernel's BTF) yourself you'd need to bpftool btf dump file /sys/kernel/btf/vmlinux format c > vmlinux.h Anyway, hopefully the ksym/weak attributes should be enough to get things working. Alan > SEC("tracepoint/syscalls/sys_enter_sendto") > int trace_sendto_entry(struct trace_event_raw_sys_enter *ctx) > { >       char buf[128]; >       int pos = bpf_strstr(buf, "A"); > } > ``` > > My kernel was initially built with CONFIG_DEBUG_INFO_BTF=n, rebuilding with > CONFIG_DEBUG_INFO_BTF=y did not change the error. > > I've only tried this with a stripped kernel image, which is 4MiB larger than > the image with no BTF info. > > Running `bpftool btf dump` on the stripped iamge does show the kfunc: > > ``` > $ bpftool btf dump file ~/git/linux-6.18.2/vmlinux | grep strstr > [26877] FUNC 'bpf_strstr' type_id=26855 linkage=static > [60337] FUNC 'strstr' type_id=60336 linkage=static > ``` > > I'm running this program in a virtual machine with a custom init; running > through strace shows a failed load of `/proc/version_signature`, but I assume a > fallback to `uname`. > > ``` > faccessat2(AT_FDCWD, "/proc/version_signature", R_OK, AT_EACCESS) = -1 ENOENT (No such file or directory) > uname({sysname="Linux", nodename="(none)", ...}) = 0 > mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f3fe6630000 > mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f3fe662f000 > mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f3fe662e000 > munmap(0x7f3fe662e000, 4096) = 0 > munmap(0x7f3fe6630000, 8192 > nanosleep({tv_sec=0, tv_nsec=10000000} > futex(0x7f3fe6a44858, FUTEX_WAKE_PRIVATE, 1) = 1 > munmap(0x7f3fe6839000, 16384 > sendto(3, "\1", 1, MSG_NOSIGNAL, NULL, 0 > futex(0x7f3fe6a43b70, FUTEX_WAIT_PRIVATE, 2, NULL > sendto(3, "@", 1, MSG_NOSIGNAL, NULL, 0) = 1 > sendto(3, "libbpf: failed to find BTF for e"..., 64, MSG_NOSIGNAL, NULL, 0) = 64 > close(3) > ``` > > Maybe there's another dependency I'm not aware of for kfuncs? > > I'm not sure what I'm doing wrong, can you point me in the right direction? > > > David > >